This invention relates to a constant voltage circuit, and particularly to a constant voltage circuit using bipolar transistors which are integrally provided in a semiconductor substrate including MOS transistors.
Recently, hand-held electronic calculators (hereinafter denoted by electronic calculator) have been widely used. On the contrary, solar cell electronic calculators using solar cells as a power supply have been already developed in conjunction with the energy saving and the saving of resources, and such electronic calculators have been brought to the market.
FIG. 1 shows a structure of a conventional solar cell electronic calculator. A circuit 1, which executes arithmetic operations depending upon entered inputs, comprises a complementary MOS LSI (hereinafter denoted by CMOS LSI). The output of the CMOS LSI is applied to an output means such as a liquid crystal display device 2, thereby displaying arithmetic results and the contents of memory. The CMOS LSI 1 and the liquid crystal display device 2 execute desired arithmetic and display operations by using the photovoltaic power fed from the solar cell 3 as a power supply.
In the case of electronic appliances using the solar cell as the power supply, they are different from appliances employing a general cell or battery as the power supply, and the generated voltage of the solar cell depends upon the illumination of light under the employed condition. Therefore, the output of the solar cell becomes a considerably high voltage under high illumination as compared with a standard illumination. If such high voltage is applied to CMOS LSI etc., normally non-lighting segments of the liquid crystal may become half lit. Consequently, it is very difficult to see the displayed contents. In addition, since the load of the LSI for operating the appliance is made high, there is still a problem that a high allowable range of operation is required for the LSI.
In order to eliminate such a problem, the solar cell electronic calculators, which have been practically applied, include a voltage limiting circuit 4 which is constituted by connecting a resistor and an LED etc. between the solar cell 3 and CMOS LSI 1 as shown by a broken line in FIG. 1.
In a driving circuit for solar cell type appliances, the voltage limiting circuit 4 using the resistor and the LED must be provided in addition to the driving circuit of LSI for the purpose of the arithmetic operation and the display. However, it is not suitable to provide the voltage limiting circuit for small electronic appliances, such as electronic calculators, electronic wrist watches and the like, with a decreased number of components, and it can not be possible to satisfy the demand for lowering the cost. In addition, it was not practical to apply the voltage limiting circuit to small appliances because sufficient space for packaging is required.